The invention relates to correction of nonlinearity caused by an I/Q modulator in a transmitter.
In new radio systems, the scarcity of radio frequencies makes it necessary to use spectrum-efficient modulation methods. In Europe, a new radio system standard has been developed for PMR (Professional Mobile Radio) users, called TETRA (Terrestrial Trunked Radio). xcfx80/4-DQPSK (xcfx80/4-shifted Differential Quadrature Phase Shift Keying) has been selected as the modulation method of the system. As far as a transmitter is concerned, a drawback to the modulation method is the variation in the amplitude of the envelope of a radio frequency signal, which causes InterModulation (IM) in a nonlinear amplifier. The IM results spread the spectrum of a transmitted signal and thus tend to reduce the gain obtained from using the linear modulation method. The IM results cannot usually be filtered since they are formed extremely close to the desired signal. With constant-amplitude modulation methods, no spreading of the spectrum occurs; therefore, the signal can be amplified by a nonlinear amplifier.
A trunked PMR system, wherein different user groups share the same radio channels, has stringent requirements regarding adjacent channel interference caused by a transmitter. These requirements necessitate good linearity in the transmitter of the radio system used.
In a power amplifier, good linearity is only achieved with poor efficiency. However, the efficiency of portable equipment should be as high as possible for the operation time to be sufficient and in order not to waste battery capacity. In addition, at least relatively good efficiency is required of power amplifiers at base stations in order to avoid cooling problems. Sufficient efficiency and linearity can only be achieved by linearizing the transmitter.
If the nonlinearities of an amplifier were known in advance, it would be possible to form inverse functions of the nonlinearities to convert the input signal, whereby the nonlinearities would be cancelled. The characteristics of the amplifier do not, however, stay the same but they change due to, for example, aging, warming up, and according to the radio channel and power level used. In addition, amplifiers have individual differences. Linearization methods are needed that are capable of adjusting adaptively to changing conditions. Research has been conducted on many different linearization methods, and three have been found to possess characteristics suitable for practical radio systems. These methods are feedforward, cartesian feedback and predistortion. A linearization method can also be adaptive.
Thus, if the nonlinear transfer function of the amplifier is known and if it does not vary as a function of time, the signal to be transmitted can be linearized by applying to the signal a suitable transfer function causing predistortion. Hence, the signal outputted from the amplifier can be made linear. This method is called predistortion. At baseband, for example, predistortion can be carried out by using a lookup table (LUT) into which are stored conversion parameters causing predistortion, i.e. predistortion parameters, in which case the conversion parameters to be used at a given time are selected on the basis of the amplitude of the signal to be predistorted.
However, the I/Q modulator of the transmitter causes an error in the system, which is due to different nonlinearity of the I- and Q-branches of the I/Q modulator. An I/Q modulator operates on a quadrature modulation basis. It enables two independent signals to be combined in the transmitter and be transmitted on the same transmission band and the signals to be separated again at the receiver. The principle of quadrature modulation is that two separate signals, I and Q (Inphase and Quadrature phase), are modulated by using the same carrier wave frequency, but the phases of the carrier waves differ from each other in that the carrier wave of signal Q lags 90xc2x0 the carrier wave of signal I. After modulation, the signals are summed. Thanks to the phase difference, the I- and Q-signals can be separated from each other when the sum signal is demodulated. Distortion caused by the different nonlinearity of the I- and Q-branches of the I/Q modulator can make measuring the nonlinearity of the amplifier difficult; therefore, the distortion is to be removed. In accordance with the prior art, the input level of the I/Q modulator is, if necessary, retained so low that potential distortion components do not affect the measurement of the amplifier harmfully. Consequently, the output signal supplied from the I/Q modulator must correspondingly be amplified more, which causes broadband noise in the system. Indeed, the I/Q modulator can be the greatest generator of noise in a chain of amplifiers. In such as case, the requirements of high linearity and low level of broadband noise are contradictory.
An object of the invention is thus to provide a method and an arrangement implementing the method so as to enable the above-mentioned problems to be solved. The objects of the invention are achieved by a method of correcting nonlinearity of an I/Q modulator in a linearization circuit of an amplifier of a transmitter, which circuit comprises an I/Q modulator for I/Q-modulating a signal to be transmitted, a predistorter for linearizing the amplifier, and a feedback arrangement for determining parameters of the predistorter of the amplifier, the method being characterized by comprising the steps of determining the nonlinearities of I- and Q-branches of the I/Q modulator, and predistorting the I- and Q-signals supplied to the I/Q modulator separately by means of the determined nonlinearities of the I- and Q-branches of the I/Q modulator in order to compensate for the nonlinearity of the I/Q modulator.
The invention is based on the idea that nonlinearities of the I- and Q-branches of the I/Q modulator are determined and compensated for by predistorting the I- and Q-signals separately.
An advantage of the method of the invention is that the I/Q modulator can be controlled using a higher level and used within a more advantageous operation range, whereby less harmful noise occurs. In addition, the correction of the nonlinearity of the amplifier is retained as a unique block in the system as the nonlinearity of the I/Q modulator is corrected by a separate block. This is preferable since it is likely that a single nonlinearity can be more readily described in, for example, a digital application than several combined nonlinearities, because a combined transfer function of several nonlinear elements more likely comprises more drastic changes than single transfer functions of the elements.
According to a preferred embodiment of the invention, nonlinearities are determined by feeding a measuring signal to the I-branch of the transmitter and a zero signal to the Q-branch thereof, sampling the I/Q-modulated signal to be transmitted, A/D-converting the signal samples taken from the signal to be transmitted, I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, determining the nonlinearity of the I-branch of the I/Q modulator on the basis of the I-feedback signal and the corresponding measuring signal fed into the I-branch of the transmitter, feeding the measuring signal into the Q-branch of the transmitter and the zero signal into the I-branch thereof, sampling the I/Q-modulated signal to be transmitted, A/D-converting the signal samples taken from the signal to be transmitted, I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, and determining the nonlinearity of the Q-branch of the I/Q modulator on the basis of the Q-feedback signal and the corresponding measuring signal fed into the Q-branch of the transmitter. Alternatively, nonlinearities can be determined by feeding a measuring signal into the I- and Q-branches of the transmitter, sampling the I/Q-modulated signal to be transmitted, A/D-converting the signal samples taken from the signal to be transmitted, I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, determining the nonlinearity of the I-branch of the I/Q modulator on the basis of the I-feedback signal and the corresponding measuring signal fed into the I-branch of the transmitter, and determining the nonlinearity of the Q-branch of the I/Q modulator on the basis of the Q-feedback signal and the corresponding measuring signal fed into the Q-branch of the transmitter.
When the feedback signal is I/Q-demodulated digitally, in which case the I/Q demodulation does not cause nonlinearity to the signal, the nonlinearity caused by the I/Q modulator can be determined by means of the feedback signals since the feedback signals comprise nonlinearity caused by the I/Q modulator only. In addition, the same feedback branch can be used for determining the nonlinearity of the amplifier and the nonlinearity of the I/Q modulator. This simplifies the structure of the predistortion system of the transmitter, and, as fewer components are needed, also saves costs.
The invention further relates to an arrangement for correcting nonlinearity of an I/Q modulator in a linearization circuit of an amplifier of a transmitter, which circuit comprises an I/Q modulator for I/Q-modulating a signal to be transmitted, a predistorter for linearizing the amplifier, and a feedback arrangement for determining parameters of the predistorter of the amplifier, the arrangement being characterized in that the arrangement comprises means for sampling the I/Q-modulated measuring signal to be transmitted, which is formed from the measuring signals fed into the I- and Q-branches of the transmitter, means for A/D-converting the signal samples taken from the signal to be transmitted, means for I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, means for determining the nonlinearities of the I- and Q-branches of the I/Q modulator on the basis of the measuring signal fed into the I- and Q-branches of the transmitter and the I- and Q-feedback signals caused by the measuring signals, and means for predistorting the I- and Q-signals supplied to the I/Q modulator separately by means of the determined nonlinearities of the I- and Q-branches of the I/Q modulator in order to compensate for the nonlinearity of the I/Q modulator. Such an arrangement enables the advantages of the method of the invention to be achieved by a simple structure.